The present invention relates generally to a test fixture for printed circuit boards. More specifically, the present invention relates to a bi-level test fixture for conducting automatic functional and in-circuit testing of printed circuit boards.
In the field of automatic testing equipment (ATE), it has been well known to employ a vacuum actuated test fixture for receiving an electronic circuit device or unit under test (UUT) to be tested and then electrically interconnecting the UUT to the automatic testing equipment. In the prior art, it has become well known for the automatic testing equipment to include a plate, commonly referred to as a probe plate, for carrying an array of test probes. Also, a board is provided for carrying the electronic circuit device or UUT to be tested, commonly referred to a diaphragm board. The diaphragm board is typically positioned above and in spring-biased relation to the probe plate. As a result, a chamber is formed between the probe plate and the diaphragm board. To actuate the diaphragm board into contact with the test probes present on the probe plate for testing, air in the chamber is evacuated to create a vacuum therein. As a result, the diaphragm board is drawn toward the probe plate for testing of the electronic circuit device. The vacuum is maintained by a seal system present between the diaphragm board, probe plate and the electronic circuit board to be tested.
Typically, test fixtures for testing printed circuit boards, operate to provide both in-circuit and functional testing. For in-circuit testing, the testing equipment is used to determine the integrity of the individual components installed on the UUT, such as determining integrity of trace paths and values of individual components. For functional testing, the cooperation of the individual components installed in the UUT is tested to determine whether the intended device function is being carried out In the prior art, it has been known to employ completely separate test fixtures for testing the UUT. However, the requirement of a separate testing unit and the significant additional labor costs make this approach cost prohibitive.
Many attempts have been made in the prior art to address the need for conducting both an in-circuit and functional test of a UUT or electronic circuit device. Test fixtures, having a first and second platen which are movable by first and second vacuum chamber actuators into contact with the electronic circuit device or UUT to selectively provide either in-circuit or functional testing, has been disclosed in the prior art.
For example, U.S. Pat. No. 4,115,735, issued to Stanford, discloses a number of probes slideably mounted in a corresponding guide that are in electrical communication by individual ones of a number of wires to the automatic testing equipment. Due to the possible leakage and vacuum loss between the first and second vacuum chamber actuators, this device is not reliable and is expensive to operate and maintain.
In addition, attempts have been made in the prior art to employ a single platen or diaphragm board which is controlled by a complex vacuum controller. In these attempts, an expensive and complex vacuum controller and system are employed to enable two different levels of vacuum to be introduced between the diaphragm board and the probe plate. The use of springs of different compression characteristics are typically employed to control the downward travel of a diaphragm board toward the probe plate when a particular vacuum level is applied. The use of additional spring members and complex vacuum controllers to deliver multiple levels of vacuum significantly increase the overall cost of the test fixture and the maintenance thereof.
Still further, attempts have been made in the prior art to use a single level vacuum source without complicated spring and platen arrangements. For example, U.S. Pat. No. 4,636,723, issued to Coffin, discloses a testing device for printed circuit boards which employs an interim spacer stop plate with protrusions thereon to stop the further downward travel of the diaphragm board to permit only the desired probes to contact the UUT for the appropriate desired test. For functional testing, the stop plate is slid horizontally into position to limit the downward travel of the diaphragm board so that only the higher functional test probes can contact the UUT. Functional testing occurs with full vacuum applied and the stop plate in its first, higher position. For in-circuit testing, the vacuum of the Coffin device must be completely released so that the stop plate may be slid into its second, lower position. A full vacuum is then applied again which draws the diaphragm board closer to the probe plate than the prior functional test so that both the functional and in-circuit test probes contact the UUT. In the Coffin '723 patent, the vacuum must be completely released to slide the stop plate which results in the functional test probes temporarily breaking contact with the UUT. Further, both functional and in-circuit testing can only occur when a full vacuum is being applied. However, the Coffin patent suffers the disadvantages of requiring that a vacuum must be present for both functional and in-circuit testing and that the functional test probes must be disengaged to switch testing modes.
Due to the demand for a test fixture which can be inexpensive to assemble, operate and maintain while still being able to provide quality functional and in-circuit testing, it is desirable for a test fixture to include a single level vacuum source and a quick and easy configuration for quickly switching between functional testing and in-circuit testing and vice versa without sacrificing the integrity of the testing cycle. It is also desirable that the test fixture employ standard parts and components and use standard vacuum sources available in most printed circuit board testing facilities.